What exactly is a biofilm? Where are biofilms found?

A biofilm is a microbial community is which the cells are encased in a layer of polysaccharide, secreted by the cells. The cells within a biofilm may or may not be of the same species. The biofilm is generally organized in such a way that water can circulate through the biofilm. Biofilms can be found on any hard surface such as rocks in a stream, pipes, or dental enamel.